Fix sending the correct pressure altitude for a pilot position

* Before this commit, only the true altitude was known for an aircraft situation. The pressure altitude was not available anywhere yet.
* This caused a wrong altitude in radar clients.
* We fix this reading the pressure altitude from the simulators and set it in the own aircraft situation.
* MS Flight Simulators have the pressure altitude in the APIs available.
* For X-Plane and emulated simulator, we need to calculate it from the pressure at sea level.
* Finally, we use the new available pressure altitude to send it to the FSD server.

Maniphest Tasks: Ref T223

src/plugins/simulator/xplane/xswiftbusserviceproxy.cpp

@@ -376,6 +376,17 @@ namespace BlackSimPlugin
         }
 
 
+        double CXSwiftBusServiceProxy::getQNH() const
+        {
+            return m_dbusInterface->callDBusRet<double>(QLatin1String("getQNH"));
+        }
+
+        void CXSwiftBusServiceProxy::getQNHAsync(double *o_qnh)
+        {
+            m_dbusInterface->callDBusAsync(QLatin1String("getQNH"), setterCallback(o_qnh));
+        }
+
+
         void CXSwiftBusServiceProxy::setCom1Active(int freq)
         {
             m_dbusInterface->callDBus(QLatin1String("setCom1Active"), freq);